Most amateurs I see spend a great deal of time focused on the break of a putt and, to a lesser extent, the speed. Those are key fundamentals for sure, but equally important is making a good stroke. Swinging the putterhead on a consistent path will help you start the ball on the line you pick.
To groove your stroke, try my two-tee drill. Find a straight putt and stand 10 feet from the cup on that line. Set the putterhead down facing the hole, and stick two tees in the green—one just outside the toe of the putter and the other just outside the heel. The tees create a gate (above) that you'll have to swing the putterhead through to keep it on a straight path to the hole.
Once you can routinely make practice strokes where the putterhead swings through the gate without touching either tee, set a ball between them and hit some putts (below). Miss the tees, and you'll start filling up the cup.
One of the reasons target shooters are so accurate is that they work on controlling their breathing. Take a cue from them when you hit putts. Make a couple of practice strokes, then address the ball. Inhale deeply and start to breathe out—but hold it about halfway and stroke the putt. I've found over the years that I putt significantly better when I breathe like this.
Excerpted from the DVD "Lessons of a Lifetime II," by Tom Watson.